Active Galactic Nuclei

Chapter 25

Revised 2012

Active Galactic NucleiCome in several varieties;

Starburst Nuclei – Nearby normal galaxies with unusually high star formation rates in the nucleus.

Seyfert Galaxies – identified by Carl Seyfert in 1943. Essentially normal, nearby galaxies, with unusually bright nuclei

Quasars – quasi-stellar objects, distant, high luminosity first recognized as galaxies by Martin Scmidt in 1963.

NGC 7742 – A Seyfert Galaxy

3C 48 – A Quasar

To learn more we have to understand the emission spectrum

A typical quasar spectrum

But, of course, it’s redshifted…

Quasar Luminosities are very high.

Quasars are observed with redshifts as high as v = 0.8c

The fact that they are so far away and so bright must mean that they have very high luminosities ~ 1011Lo which is 1000 timesmore luminous than a normal spiral galaxy.

The most distant quasars have the largest redshifts and very broad

emission lines.

We also see evidence for Jets

Jets power lobes of radio emission

A closer look at the nucleus reveals disks

All of which leads to a standard model for AGN’s

Time Variability

Some quasars are observed to change in brightnesson timescales of days, which can be used to set a limit on the size of the emitting region.

Correlating the continuum variations with the emission line variations provides the light travel time t which yields the size of the emitting region

r ~ c t

r

Variability observed on a timescale of a day leads to the following size; r = 3 x 108 m/s 24 hrs/day 60min/hr 60s/min

r ~ 1013 – 1014 m or 100 – 1000 AU !

and the broad emission lines are radiated from the disk which allows an estimate

of the black hole mass

v

MBH rm

Balancing forces;

mv2/r = G MBH m /r2

which re-arranges to MBH = 2.32 x 105 v2(km/s) r(kpc) Mo

If the width of the broad lines, typically 10,000 km/s wide, reflects the rotational velocity of an accretion disk around a super massive black hole, then the line width together with a size for the emitting region leads to an estimate for the mass of the black hole, typically,

MBH ~ 108 M

The large mass inferred from the broad lines combined with the small size inferred from the rapid time variabilitycombined with the high luminosities inferred from the largeredshifts all point to an exotic object at the heart of a quasar - a Massive Black Hole

Are the broad lines caused by a disk or an inflow?

Modeling the shape of the broad line also provides a measure of thesize for the ionized region emitting the H emission lines.

Inflow – the Movie!

Basic Black Hole PhysicsEscape Velocity

In the case of a Black Hole, the maximum escape speed is c, the speed of light, so

c2 = 2 GMBH/R

which can be re-arranged to make R the subject of the formula

R = 2GMBH/ c2 also known as the Schwarzschild radius or “the event horizon”

The Schwarzschild radius for a typical quasar is

R = 2GMBH/ c2

R = 2 . (6.67 x 10-11) 108 (1.99 x 1030 )/ (3 x 108)2

R = 2.94 x 1011 m or ~ 2 AU for a 108 Mo Black Hole

Energy SourceIt is the release of gravitational potential energy as matterfalls into a black hole that drives the high luminosities observedfrom quasars

Quasar Evolution

Quasars are observed only at high redshifts – at largelookback times, so they existedin the distant past. There areno nearby quasars.

The lower luminosity AGN’s,the Seyfert and starburst galaxies, bridge the gap between us and the more distant quasars

Galactic Center- Black Hole

Ghez, A, 1998, ApJ...509..678 MBH = (2.6 +/- 0.2) x 106 M

Tidal disruption

Stars passing close to a BH are torn apart by the enormous gradient in the gravitational field strength between the front-side of the star

facing the BH and the back-side of the star furthest from the BH.